1. Field of the Invention
The present invention relates to an auto gain controller, and more particularly, to an auto gain controller having a temperature compensation function.
2. Discussion of the Related Art
A conventional auto gain controller having a temperature compensation function will be described with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a conventional auto gain controller having a temperature compensation function. As shown in FIG. 1, the auto gain controller includes a dual gate field effect transistor Q1 and a gain amplifier 11. A source voltage of the dual gate field effect transistor Q1 is applied to a second gate G2 through the gain amplifier 11.
When a gain is reduced due to an increase of the peripheral temperature, a drain current of the transistor Q1 is reduced, thereby reducing the source voltage of the transistor Q1. The reduced source voltage is applied to an inversion terminal of the gain amplifier 11. An output signal of the gain amplifier 11 is applied to the second gate G2 of the transistor Q1. Consequently, the drain current of the transistor Q1 increases and thus the gain of the transistor Q1 which compensates for the temperature variation increases.
FIG. 2 is a circuit diagram according to one embodiment of the conventional auto gain controller. As shown in FIG. 2, the conventional gain controller includes a receiving part 21 and a transmitting part 22.
The receiving part 21 includes a dual gate field effect transistor Q1, a gain amplifier 21b, and a differential amplifier 21a for comparing a detecting signal with a reference signal. Likewise, the transmitting part 22 includes a dual gate field effect transistor Q2 and a gain amplifier 22a.
The detecting signal of the receiving part 21 is compared with the reference signal Vref by means of the differential amplifier 21a. The resultant output signal is applied to a non-inversion input terminal of the gain amplifier 21b.
The source voltage of the field effect transistor Q1 is applied to an inversion input terminal of the gain amplifier 21b and an addition circuit 23. An output signal of the addition circuit 23 is applied to a non-inversion input terminal of the gain amplifier 22a of the transmitting part 22.
An output signal RX-V.sub.GAIN CONT of the gain amplifier 21b is applied to the second gate G2 of the transistor Q1.
The source voltage of the transistor Q2 in the transmitting part 22 is applied to the inversion input terminal of the gain amplifier 22a. An output signal of the gain amplifier 22a becomes TX-V.sub.AGC, i.e., AGC voltage.
In the receiving part 21, the second gate G2 and the source of the transistor Q1 and the gain amplifier 21b are realized by a closed loop. Thus, temperature variations are compensated by the transistor Q1.
Likewise, in the transmitting part 22, the source and the second gate G2 of the transistor Q2 and the gain amplifier 22a are realized by a closed loop, so that temperature variations are compensated by the transistor Q2.
The temperature compensated signal from the transistor Q1 of the receiving part 2l is provided to the gain amplifier 22a of the transmitting part 22 through the addition circuit 23. Thus, the gain variation of the amplifier due to the peripheral temperature variation is reduced. The transmitting power source is accurately controlled by the receiving signal level. This prevents speech quality from being deteriorated due to variation of the transmitting power source.
However, the aforementioned conventional auto gain controller has several problems. If noise occurs when the receiving signal is input to the receiving part, the drain current of the transistor Q1 varies, thereby causing noise to occur in the output signal. In other words, the drain current of the transistor Q1 varies in accordance with the variation of the input signal, so that the current which flows to the gain amplifier also varies. As a result, the output signal becomes nonlinear.